Interfering with time can be tricky. The Elvish can do this within their own world very easily, but the laws of the Physical world requires cooperation between all parties - which in and of itself takes more time again to manage also.
Only an adept can understand and convene with those involved in such a chaotic scene - its a little like unravelling a rug of knitted squares, and knitting them back up again, without having a fibre out of place when all is done.
The essential part for success is procuring consent. The young mother and her child needed to want to come back into the World for this to work at all. If had they been content to stay in the spiritual worlds, Puck could not have made the difference as he did.
Equally, if Eve herself had wanted the accident to play out as it had happened, he would have difficulty changing her role as well.
-Gabriel Brunsdon, Finding Self - Second Guesses- Azlander Series